Jeremy Corbyn is drawing up a new-look Labour frontbench team after his seismic victory in the party's leadership race sparked a flurry of senior resignations.
The veteran left-winger won a landslide victory with almost 60% of the vote - trouncing mainstream rivals Andy Burnham, Yvette Cooper and Liz Kendall.
He topped the ballot not just among new supporters paying £3 to join and trade unionists but also full party members, making it extremely difficult for any disgruntled moderates to mount a bid to unseat him before the 2020 general election.
In an acceptance speech to cheering supporters at Westminster, he called repeatedly for "unity" and announced his ambition to lead a Labour "fightback".
But the scale of his task in bringing the party together around a radical programme was underlined by the immediate departure of several frontbenchers.
Ms Cooper was joined by Rachel Reeves, Emma Reynolds and Jamie Reed in announcing they would not serve on the frontbench while Ms Kendall, Tristram Hunt and Chris Leslie had already indicated their political differences with Mr Corbyn.
Ed Miliband - whose resignation after leading the party to general election disaster in May provoked the contest - called on the party to join him in supporting Mr Corbyn but indicated that he too would not seek a return to the frontline.
However, shadow business secretary Chuka Umunna - a senior centrist who withdrew from the contest after a matter of days - issued a plea for the party to "come together" behind its new leader "and focus on providing the most credible and effective opposition to the Tories".
Corbyn supporters chanted "Jez we did" and sang the socialist anthem The Red Flag as they celebrated his comprehensive victory, taking 59.5% of the vote - 251,417 of the 422,664 votes cast - against 19% for Mr Burnham, 17% for Ms Cooper and 4.5% for Ms Kendall.
The new leader has spent his entire 32-year parliamentary career on the backbenches and entered the leadership contest as a 200-1 outsider before generating a wave of enthusiasm which swept him to victory.
He sought to galvanise that support first in a victory party with activists at a Westminster pub and then by addressing - as his first official engagement as Opposition leader - a huge crowd attending a pro-refugee rally in Parliament Square.
Now, he is familiarising himself with the party's nearby HQ as he starts to piece together a shadow cabinet able to deliver his anti-austerity, anti-war policies without splitting the party.
Union leaders welcomed his victory but Conservative Defence Secretary Michael Fallon said a Corbyn-led Labour posed "a serious risk to our nation's security, our economy's security and your family's security".
West Bromwich East MP Tom Watson was elected deputy leader and is expected to play an important role in helping to build Mr Corbyn's frontbench team and maintain party unity and discipline.
Mr Corbyn admitted he was "a bit surprised" at the scale of his majority, which he said amounted to a "fantastic mandate for change in British politics, with a fantastic enthusiasm for real democratic politics".
Asked whether he now faces a challenge to construct a shadow cabinet without several senior figures who have said they will not join, the new Labour leader said: "There's going to be an inclusive, open process.
"I hope everyone will recognise the mandate we've received and that party members expect our party to deliver for them in Parliament."